Forecast Update
Valid: Fri 26 Aug 2011 11:00 to Sat 27 Aug 2011 06:00 UTC
Issued: Thu 25 Aug 2011 11:50
Forecaster: GROENEMEIJER

+ + + CORRECTED TEXT at 12:05 UTC + + +

A level 2 was issued for NW Germany, Denmark and extreme NE Netherlands for widespread damaging winds, and for large hail and tornadoes.

A level 2 was issued for W and central Germany and extreme NE France for damaging winds and for large hail.

A level 2 was issued for SE France, mainly for heavy rain.

A level 1 was issued for the remainder of Denmark and almost all Germany and for E, central and S France, Switzerland, and for the for severe wind gusts and large hail.

DISCUSSION

N Germany, Netherlands, Denmark

A level 2 was retained across NW and N Germany, NE Netherlands, and parts of Denmark because of ongoing convective initiation in a strongly sheared and unstable environment. Winds at 700 hPa of around 25 m/s and about 2000 J/kg of CAPE provide an environment supportive of bowing systems (as already observed across central and SE Netherlands) and supercells. With supercell storms, tornadoes may develop.

W and central Germany, NE France

Have decided to upgrade W and central Germany to level 2, because initiation of convection along and just ahead of the cold front has become more likely per latest numerical guidance (GFS, ECMWF etc.). Given the strong background flow at 700 hPa and the large temperature contrast across the front, damaging wind gusts are a particular concern. It appears that several bowing features will travel NNEward across a zone roughly covering the western third of Germany, NE France and the E and NE Benelux.

More conditional, but still marginally warranting a level 2 is the severe wind threat further east across Germany. If convection manages to form along the front, damaging winds will be the main threat here as well, although large hail is quite possible with any more isolated storms.

SE France

Have upgraded parts of the western Alps and lower Rhone Valley because of flash flood threat due to expected back-building storms in this area, fed by a particularly moist northward flow off the Mediterranean. Amounts of 50-100 mm may locally accumulate in little time.